A drilling riser is made up of an assembly of tubular elements whose length ranges between 15 and 25 m, assembled by connectors. The weight of these risers borne by an offshore platform can be very great, which requires suspension means of very high capacity at the surface and suitable dimensions for the main tube and the linking subs.
So far, the auxiliary lines: kill lines, choke lines, booster lines and hydraulic lines are arranged around the main tube and they comprise subs that fit together, fastened to the riser element connectors in such a way that these high-pressure lines can allow a longitudinal relative displacement between two successive line elements, without any disconnection possibility however. Owing to these elements mounted sliding into one another, the lines intended to allow high-pressure circulation of an effluent coming from the well or from the surface cannot take part in the longitudinal mechanical strength of the structure consisting of the entire riser.
Now, in the perspective of drilling at water depths that can reach 3500 m or more, the dead weight of the auxiliary lines becomes very penalizing. This phenomenon is increased by the fact that, for the same maximum working pressure, the length of these lines requires a larger inside diameter considering the necessity to limit pressure drops.
Document FR-2,799,789 aims to involve the auxiliary lines, kill lines, choke lines, booster lines or hydraulic lines, in the longitudinal mechanical strength of the riser. According to this document, a riser element comprises a main tube, connecting means at both ends thereof, at least one auxiliary line length arranged substantially parallel to the main tube. The auxiliary line length is secured at both ends to the main tube connecting means so that the longitudinal mechanical stresses undergone by the connecting means are distributed in the tube and in the line.
One difficulty in making the riser according to document FR-2,799,789 lies in the fastening means for joining the auxiliary line length to the main tube. The tensional stresses undergone by the auxiliary line length are transmitted by these fastening means. The assembly and design requirements impose a distance to be provided between the main tube and the auxiliary line. This distance acts as a lever arm for the tensional stresses transmitted to the auxiliary line. As a result of the tensional stresses associated with the lever arm, the fastening means are subject to bending strains that may be harmful to the good working order of the riser.
The present invention provides a riser made according to a principle that is an alternative to the principle disclosed by document FR-2,799,789. According to the present invention, all of the auxiliary lines contribute, together with the main tube, to taking up the longitudinal stresses applied to the riser.